CP/M 3, also known as CP/M Plus, was released in 1983. CP/M 3 is the last version of CP/M written for 8-bit processors like the 8080 and Z80. By the time of this release, the IBM PC and MSDOS had been out for over a year and soon took over the personal computer marketplace. For this reason, CP/M 3 never reached the market penetration CP/M 2.2 achieved during the preceding years.

By 1983, about five years had passed since the last Altair 8800 was manufactured, so it’s unlikely any commercial distributions of CP/M 3 for the Altair were developed. Further, I have not found a hobbyist with a CP/M 3 disk configured to run on the Altair, so I decided to create one myself.

The Altair CP/M 3 BIOS supports the DEVICE feature of CP/M 3 (very similar to the IOBYTE feature in CP/M 2.2). Valid port assignments include both ports on the 88-2SIO at I/O addressess 10h and 12h, the 88-SIO port at I/O address 0, and the cassette interface (ACR) port SIO at I/O address 6.